Carbon dioxide as a temporary protective agent for highly efficient and selective access to diols via KI-catalyzed hydration of epoxides

The utilization of CO₂ as a protective agent to adjust and control the reactions for the highly selective synthesis of fine chemicals was rarely studied. Herein, a strategy of CO₂ as a temporary protective agent was disclosed and well applied to the KI-catalyzed hydration of epoxides to synthesized diols with high yields (87.8–98.9%) and selectivities (89.5–99.1%) at a low H₂O/epoxide ratio of 1.5:1. Isotope labeling experiments using isotope-labeled water (H₂¹⁸O) confirmed that the strategy remained direct hydration of epoxides. Mechanistic experiments and density functional theory (DFT) calculations implied that CO₂ as a temporary protective agent, protected the oxygen-negative intermediates formed from the ring opening of epoxides to generate carbonate anion, thereby safeguarding the hydration of epoxides against potential condensation reactions and simultaneously activate H₂O for nucleophilic substitution reaction. Finally, CO₂ could be automatically released later through the decarboxylation process. The strategy with excellent functional group tolerance, the viability from the gram-scale reaction, and the flue gas reaction will open up new avenues for applications of CO₂ in the hydration of epoxides and inspire novel synthetic design by utilizing CO₂ as a temporary protective agent.